6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



nucleus may or may not be present. It is evident also that different 

 nodules may have been formed in different ways, and hence we shall 

 confine our conclusions to the limits of our observations. 



4. In the summary we shall discuss the bearing of this study on the 

 depth and condition of deposition of the chalk of the surroundiug 

 formations. 



Organisms found in the Flint. 



Slides taken from the same nodule were found to present great simi- 

 larity of kinds of organisms, but great difference in size. A species of 

 spicule, for instance, is often found in all stages of development in the 

 same slide. Different nodules, however, show great variation in the 

 prevailing kinds of organisms and the condition of their preservation. 

 On the whole, however, the number of organic remains is few, and the 

 massive silica greatly in excess. When viewed with high power of 

 microscope, this massive portion appears often to consist of aggregations 

 of minute granules varying in color from brown to almost colorless. 

 Dr. Hinde 1 found the same true in cherts of the Greensaud, and speaks 

 of the granules as having " circular outlines, though not strictly a 

 spherical form." Chalcedonic masses with their characteristic concentric 

 structure are common in some of the nodules and rare in others. Many 

 traces of organisms, the outlines of whiGh were but dimly shown, were 

 not made out ; but those determined were of the groups given below. 



1. Foraminifera. These were found in every slide examined, and 

 were quite numerous in one of the nodules. The types found were 

 principally of the family Globigerina, of which several different species 

 were common. Textularia were found in most of the slides, though not so 

 common as Globigerina, and several other families were found occasion- 

 ally. These forms were generally replaced by amorphous silica, the 

 outlines remaining in a dark substance which has the appearance of 

 organic residue. Globigerina were also found replaced in chalcedonic 

 silica, but the outlines and structure of the chalcedonic mass almost 

 obliterated the form of the fossil. 



2. Sponges. In two of the nodules the spicules were of frequent 

 occurrence, but not very abundant ; but in one nodule of which I had 

 two sections, not a distinct trace was found, except occasionally a small 

 fragment of the spicule of a Monactinellid shown in Figure 7. The mi- 

 nute dermal spicules are, generally speaking, better preserved than the 



1 Sponge Remains of Lower ami Qppei Greensand, Phil. Trans. Royal Society, 

 Part II., 1885, p 127. 



